Quality of light emitted by a vehicle headlight is important with respect to road safety. Brightness and light distribution may depend on quality of the light source, quality of imaging optics, absolute and/or relative positioning of the imaging optics or the headlight and relative positioning of the light source with respect to the imaging optics. Especially the light distribution on the road or street is essential for a driver of a vehicle to recognize dangers as soon as possible, but without glare of the oncoming traffic. There are thus a number of legal regulations describing quality criteria of light sources used in vehicle headlights (e.g. Economic Commission for Europe (ECE) Regulation (R) 37 in Europe) and regarding the light distribution of a vehicle headlight comprising such a light source (e.g. ECE R112 in Europe). Both regulations mentioned above are part of the Agreement Concerning the Adoption of Uniform Technical Prescriptions for Wheeled Vehicles, Equipment and Parts which can be fitted and/or be used on Wheeled Vehicles and the Conditions for Reciprocal Recognition of Approvals Granted on the Basis of these Prescriptions. These regulations refer to the performance of a vehicle headlight or a light source comprising the vehicle headlight at the time of sale.
The dissertation of Henning Kiel “Bewertung von Kraftfahrzeugscheinwerfern mit Lichtsimulation” defended on 11 May 2015 at the university of Ilmenau, Germany gives a systematic assessment in particular of the definition of the bright/dark cutoff line of a low beam light as given in these regulations. The dissertation addresses the challenges and deficiencies of the manual aiming of low beam headlights as still largely employed in practice and proposes automatic methods based on light measurements and mathematical algorithms to arrive at more reproducible results and allowing full automation of aiming tasks. Similar issues are addressed also in the article “Tomasz Targosinski: Importance of Cut-Off Line in Automotive Headlamps Aiming. Journal of KONES Powertrain and Transport, Vol. 19, No. 3, 2012” and in the report “Michael Sivak et al.: Visual Aiming of European and U.S. Low-Beam Headlamps. Report No. UMTRI-91-34, University of Michigan, November 1991” (the latter also published under SAE TECHNICAL PAPER SERIES, vol. 1, 24 Feb. 1992, US ISSN: 0148-7191, DOI: 10.4271/920814).
In view of aging of light sources, misalignment of light sources and potential damages of, for example, imaging optics there is a need to check quality of light and to give advice to a driver to keep lighting quality of a vehicle headlight in line with the defined criteria over lifetime of the vehicle.